Semiconductor technology and chip manufacturing advances have resulted in a steady decrease of chip feature size to increase on-chip circuit switching frequency (circuit performance) and the number of transistors (circuit density). Shrinking/reducing device or field effect transistor (FET) feature sizes and, correspondingly, device minimum dimensions including horizontal dimensions (e.g., minimum channel length) and vertical dimensions (e.g., channel layer depth, gate dielectric thickness, junction depths and etc.) shrinks device size for increased device density and device performance, as well as reduces device operating conditions, i.e., chip and correspondingly, device supply voltages and voltage swings. Generally, all other factors being constant, the active power consumed by a given unit increases linearly with switching frequency, i.e., performance. Thus, not withstanding the decrease of chip supply voltage, chip power consumption has increased as well. Both at the chip and system levels, cooling and packaging costs have escalated as a natural result of this increase in chip power. For low end systems (e.g., handhelds, portable and mobile systems), where battery life is crucial, reducing net power consumption is important but, such a power reduction must come without degrading chip/circuit performance below acceptable levels.
To minimize semiconductor circuit power consumption, most integrated circuits (ICs) are made in the well-known complementary insulated gate FET technology known as CMOS. A typical CMOS circuit includes paired complementary devices, i.e., an n-type FET (NFET) paired with a corresponding p-type FET (PFET), usually gated by the same signal. Since the pair of devices have operating characteristics that are, essentially, opposite each other, when one device (e.g., the NFET) is on and conducting (modeled simply as a closed switch), the other device (the PFET) is off, not conducting (ideally modeled as an open switch) and, vice versa. Thus, ideally, there is no static or DC current path in a typical CMOS circuit and ideal CMOS circuits use no static or DC power and only consume transient power from charging and discharging capacitive loads.
A CMOS inverter, for example, is a PFET and NFET pair that are series connected between a power supply voltage (Vdd) and ground (GND). Both are gated by the same input and both drive the same output, typically a capacitive load. The PFET pulls the output high and the NFET pulls the output low at opposite input signal states. Ideally, when the gate of a NFET is below some positive threshold voltage (VT) with respect to its source, the NFET is off, i.e., the switch is open. Above VT, the NFET is on conducting current, i.e., the switch is closed. Similarly, a PFET is off when its gate is above its VT, i.e., less negative, and on below VT. Similarly, a typical CMOS storage cell, such as a static random access memory (SRAM) cell includes a pair of cross coupled such inverters as a storage latch and a pair of pass gates attached to the inverters for reading and writing the cell. When one of the inverters is driving high, the other is driving low (e.g., latching a 1) and vice versa (e.g., latching a zero). An ideal SRAM cell conducts no DC current through either of the cross coupled inverters or through either of the pass gates and holds its current state until it is written over, i.e., turning on the pass gates and forcing the opposite state.
In practice, typical FETs are much more complex than switches and transient power for circuit loads accounts for only a portion of CMOS chip power consumption. FET drain to source current (DC current and so, DC power consumed) is dependent upon circuit conditions and device voltages. Especially since device VT is directly proportional to gate dielectric thickness, as FET features (including gate dielectric thickness) shrink, off FETs conduct what is known as subthreshold current, i.e., at gate biases below threshold for NFETs and above for PFETs. Further, for a particular device, subthreshold current increases exponentially with the magnitude of the device's drain to source voltage (Vds) and reduces exponentially with the magnitude of the device's VT. This is especially true in what is known as partially depleted (PD) or fully depleted (FD) silicon on insulator (SOI) technologies, where subthreshold leakage has been shown to increase dramatically, such that it may be the dominant source of leakage. Additional device leakages including gate leakages (i.e., gate to channel, gate to source or drain and gate induced drain leakage (GIDL)) and source/drain junction leakages also contribute to static power.
When multiplied by the millions and even billions of devices on a state of the art SRAM, even 100 picoAmps (100 pA) of leakage in each of a million cells, for example, results in chip leakage on the order of 100 milliAmps (100 mA). Thus, as SRAM chip features have shrunk, these leakage sources have become more prominent. It has become especially difficult to scale gate dielectric simply because gate dielectric thickness is reaching its limit. At a few mono-layers of Silicon, for example, deposition non-uniformity causes pinholes and dielectric porosity becomes a problem dramatically lowering yield. Pinholes and dielectric porosity can cause catastrophic failures such as gate to channel shorts that develop over time with use. Further, deposition non-uniformity can cause device to device threshold variations that may be intolerable, e.g., where a matched pair of devices are needed. Moreover, supply voltages cannot be scaled appropriately, e.g., because of severely reduced device drive, especially for PFETs, from device threshold instability over time and moderate mobility gains. So, as the leakage is increasing, device drive characteristics are falling off and the leakage has resulted in degraded performance and reduced device functionality, not to mention reduced circuit noise immunity and stability. Generally, approaches to increasing device VT to mitigate subthreshold leakage, e.g., with thicker gate dielectric or back biasing device channels for example, have been applied uniformly across all circuits on a chip. Especially for complex chips and arrays with a large number of devices, device leakage (both gate and subthreshold) chip leakage power can be overwhelming, but leakage reduction techniques are equally unpalatable. So unfortunately, leakage and gate dielectric limits have become constraints on performance and chip density.
Thus, there is a need for improved SRAM cell stability and reduced cell leakage with minimal performance degradation and in particular for maximizing device off resistance while minimizing device on resistance, especially for PD SOI IC chips.